ABSTRACT Support is requested for a Keystone Symposia conference entitled Maternal-Fetal Crosstalk, organized by Drs. Sing Sing Way, Carole R. Mendelson and Michal A. Elovitz. The conference will be held in Keystone, Colorado from June 7-11, 2020. An interwoven array of physiological changes required for sustained growth and protection of the developing fetus occurs during pregnancy. Despite its necessity for species survival, many important aspects of how pregnancy is maintained are still undefined and reproductive biology remains an understudied area of biomedical research. For example, there are gaps in the knowledge pertaining to how pregnancy duration and birth timing are regulated, basic questions on how the placenta develops and functions, and on symbiotic interactions between genetically foreign maternal and fetal tissues with commensal microbes that exist at the maternal-fetal interface. With such gaps in knowledge unresolved, it is not surprising that pregnancy complications remain the leading cause of infant and childhood mortality. The primary goal of this conference on maternal-fetal crosstalk is to bring together scientists and trainees with specialized expertise in unique aspects of pregnancy to accelerate the research behind these important knowledge gaps. Emerging and controversial aspects of pregnancy physiology and pathogenesis will be discussed in the collegial spirit that is a defining hallmark of Keystone meetings. Given the direct links between biological and immunological mechanisms underlying the maintenance of pregnancy and the health of infants and children, this conference will be held jointly with Immunity in Early Life: From the Maternal to the Microbial Environment. The conferences will share sessions on maternal-fetal immunological tolerance and how primary colonization of offspring influences development.